Crude oil contains a broad mixture of different sizes and types of hydrocarbons, such as C2-C50 hydrocarbons, or even higher. Different classes of hydrocarbons are suitable for making different end products with different uses. Accordingly, crude oil is typically first refined by atmospheric distillation to produce portions or “fractions” of hydrocarbons including light distillates, middle distillates, and heavy distillates and residuals. Each fraction contains a mixture of hydrocarbons having a narrower range of properties than the original crude oil mixture. The operating conditions under which atmospheric distillation is performed can shift the quantity and particular property ranges of each fraction that is produced. Due to this variability, there is some overlap of the property ranges among the various fractions.
For example, the light distillate fraction includes primarily C2-C10 hydrocarbons, which are useful for making products such as liquid petroleum gas (LPG), gasoline, and naphtha. Middle distillates include primarily C8-C23 hydrocarbons, which are the mid-sized hydrocarbon molecules from the original crude oil and are useful, for example, as kerosene, diesel fuel, jet fuel, and light fuel oil. The heavy distillate fraction and residual products derived from atmospheric distillation of crude oil include primarily C18 and higher hydrocarbons, which are useful, for example, as heavy fuel oil (HFO), lubricating oils, wax and asphalt. It is understood that each distillate fraction may, in fact, contain very small amounts of hydrocarbons outside the aforesaid ranges, without altering the general characteristics of the distillate fractions.
Further refining and processing of the various atmospheric distillation fractions is often performed to produce fractions having even narrower characteristics. For instance, light fuels such as kerosene, diesel fuels, and heavier distillate fuels may be obtained by further fractionation of the middle distillate fraction. Additionally, the heavy distillate and residual fraction is often further separated into heavy fuel oils, lubricating (lube) oils, waxes and asphalt (bitumen) products. Such separation is often accomplished, for example, by vacuum distillation which in addition to producing the aforesaid products from the heaviest atmospheric distillation fraction, also produces vacuum gas oil (VGO). In turn, hydrocracking of VGO converts these relatively heavy hydrocarbons to mid-range and light hydrocarbons, with a portion of the heavy hydrocarbons leaving the hydrocracker as unconverted oil (UCO). VGO and UCO are commonly further refined and processed, e.g., using hydrocracking, hydrotreating, and separation methods, to produce various products including some distillate fuels and high quality lube basestocks.
Both distillate fuel oils and high quality lube basestocks can be, and are routinely, manufactured from the same or similar classes of heavy hydrocarbons, including VGO and UCO. However, different operating conditions and different catalyst compositions are typically employed to make these two different petroleum products, which generally means that separate process apparatus is used to produce them.
Accordingly, it is desirable to provide methods and systems that enable production of one or the other product, alternately, using the same refining system with minimal adjustments between operational modes (i.e., fuel mode or lube mode). Other desirable features and characteristics of the methods and systems described herein will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings.